1. Field of the Invention
The present invention relates to an optical entire-circumference encoder and a motor system.
2. Description of the Related Art
To measure physical amounts such as the position, velocity, and the like of a mobile body, an encoder is used.
The encoder is roughly divided into mainly a rotary type (hereinbelow, also simply called “rotary encoder”) and a linear type (hereinbelow, also simply called “linear encoder”) in accordance with the movement direction of a mobile body.
A rotary motor (hereinbelow, also simply called “motor”) using a rotary encoder (hereinbelow, also simply called “encoder”) is often subjected to position control based on a rotation position or the like detected by the encoder. The precision or the like of such position control is largely influenced by the precision of the encoder. Therefore, in devices and the like which are becoming more precise, a higher-precision encoder is being developed for higher-precision position control.
Particularly, an optical encoder among encoders roughly divided to various kinds in accordance with detection principles can realize higher-precision position detection as compared with the encoders using other detection principles such as the magnetic type. A main optical encoder irradiates slits formed in a disc connected to a rotor (an example of a mobile body) with light and receives reflection light or transmission light from the slits. As a result, the optical encoder detects a position based on the reflection light or transmission light repeated in accordance with the rotation of the disc. Therefore, the optical encoder can realize very high position detection precision in accordance with the precision of forming the slits.
In such an optical encoder, to generate reflection light or transmission light corresponding to rotation of a disc, fixed slits corresponding to rotation slits formed in the disc are often used. Specifically, the optical encoder is constructed so that, when a rotation slit reaches a predetermined position with respect to a fixed slit in accordance with rotation of a disc, reflection light or transmission light reaches a light receiving element. Therefore, the light receiving element receives a signal according to the rotation of the disc and generates position information from the light reception signal. Thus, in the optical encoder, to perform high-precision position detection, a positional relation between the rotation slits and fixed slits has to be adjusted with high precision for the following reason. In the case where there is an error in the positional relation between the fixed slits and the rotation slits, noise increases due to reception of reflection light and transmission light which is not desired in designing by the light receiving element by the amount of the error.
To reduce such noise, a rotary encoder of an optical entire-circumference correction type (hereinbelow, also called “optical entire-circumference encoder”) has been developed (refer to, for example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-515426). The optical entire-circumference encoder irradiates almost all of a plurality of slits formed in the entire circumference of a disc with light and receives the reflection light or transmission light. Therefore, even in the case where an error occurs in the positional relation between a fixed disc and a rotating disc such as a case where a disc is attached eccentrically, by using reflection light or transmission light obtained from the entire circumference, the error can be cancelled out. Therefore, the resistance to such an error is increased as described above and, as a result, manufacture of the optical entire-circumference encoder can be facilitated.
On the other hand, although not limited to an optical encoder, to detect the rotation direction of a disc, two or more periodical light reception signals whose phases are different from each other by, for example, 90 degrees in electric angle have to be obtained. The two light reception signals whose phases are different from each other will be also called an A-phase signal and a B-phase signal.
To generate such an A-phase signal and a B-phase signal, in the optical entire-circumference encoder, as described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-515426, at least one of the rotation slits and the fixed slits formed in the entire circumference around the rotary shaft is formed by double slits whose phases are different from each other by 90° and which are arranged in the radial direction. An optical path or the like is doubled in the radial direction or the height direction (thrust direction) so as to obtain different signals every doubled slits. The optical path and the like doubled in the radial direction or the height direction is a cause of enlarging the size of the entire apparatus.